In the fabrication of optics, magnetic disks, micromachines, and particularly semiconductor integrated circuit devices, there is a desire to perform chemical mechanical planarization (CMP) operations. Specifically, integrated circuits are fabricated in the form of multi-level structures on semiconductor wafer substrates. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. Traditionally, suitable metal films included aluminum and its alloy as well as tungsten and the associated barrier and liner films for each. More recently, Cu metallization along with its associated barrier, liner and passivation films has emerged and is rapidly becoming the conductor of choice for advanced device fabrication. Similarly, insulating materials have evolved with traditional silicon dioxide taking a back seat to more advanced materials with lower dielectric permittivity. A dominant trend within this technology advancement is the migration to smaller and smaller transistor features and the more extensive layer stacking of multilevel metallization required to connect to a more highly packed transistor plane.
As the device side shrinks, the lithography requirement as well as the requirements of many of the other process steps becomes more stringent. The ability to create more metallization levels and associated dielectric layers requires increasingly more planar surfaces. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. CMP has emerged as a dominant planarization process for advanced microelectronics and is used for both the planarization of metal and dielectric layers. Furthermore, the in-laid metal Damascene approach, which has been accomplished in large part due to the CMP process, has been a dominant approach to form interconnect structures and to enable improved circuit performance, fabrication line manufacturability and process line and device yields.
CMP has the capability to achieve significant planarization efficiency across many orders of magnitude length scale. In the most advanced wafer-scale technologies, wafers 300 mm in diameter are polished. The planarization that is achieved at this length scale is generally termed the ‘within-wafer-nonuniformity’ or ‘across-wafer-nonuniformity’. At the opposite end of the spectrum, CMP must provide planarization capability at the atomic scale or Angstrom length-scale, and is generally termed ‘surface roughness’. Intermediate length scale performance is also required. Thus, planarization must be achieved across many orders of magnitude length scale and CMP has emerged as the dominant technique in this regard.
A chemical mechanical planarization (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. More advanced systems also include modules to perform post-CMP cleaning of the wafer surfaces. In the CMP Process, a wafer is typically pressed against a polishing surface (polishing pad) in the presence of a steadily supplied polishing liquid and the lateral motion of a polishing surface relative to wafer surface imparts mechanical energy which in combination with the chemical nature of the liquid, acts to effect surface removal. CMP equipment designs have incorporated rotary, orbital as well as linear system motions. Machine configurations leveraging other motions are viable as well. A well engineered CMP process leverages the consumables (pad, slurry, etc.) and process recipes on a particular platform to achieve an overall CMP process. Application of this process achieves the preferential removal of protruding surface features and the preferential nonremoval of recessed features and in this way surface smoothing and planarization is achieved. The relative extent of removal of protruding vs. recessed features determines the planarization efficiency.
The polishing liquid referenced above is typically loaded with abrasive and in such cases is termed slurry. In the absence of any abrasive, the polishing liquid has also been termed either simply a reactive liquid or an abrasive-free slurry. In general, the slurry can be viewed as a polishing liquid in which a chemical mixture is loaded with an amount of abrasive. The chemical mixture can include a wide range of chemical components including, but not limited to, chemical ligands, chelating and complexing agents, corrosion inhibitors, pH adjusting acids and bases, pH buffers and surfactants. Abrasives can be made of a wide range of materials, formed from a wide range of processes and incorporated over a wide range of content including, but not limited to, organic particles and inorganic particles, such as SiO2, TiO2, and Al2O3 formed by fuming or solution growth from well below 0.1% to above 40% by weight. In general, it is desirable to maintain the abrasive as a colloidal suspension to improve defectivity and minimize slurry handling issues.
The polishing article is typically called a polishing pad. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad material itself is typically based upon polyurethane; though a wide range of other pad materials are viable. Both open and close microcellular pads, noncellular pads, fiber pads of both woven and nonwoven construction, filled pads employing embedded abrasive, and coated pads with ceramic surface layers, have all been applied in semiconductor CMP processing. Important pad characteristics which influence CMP performance include, but are not limited to, material composition and micro structure, surface macro, micro and nanotexture, pad layer stacking, pad material properties including mechanical properties of hardness, elastic, inelastic and anelastic behavior, and surface tension.
As a result of the pad and slurry consumables used and the process parameters employed in CMP processes, a phenomenon known as ‘glazing’ occurs in which the process performance drifts as a result of an inherent instability of the process. Typically, the material removal rate and associated across wafer rate uniformity drift outside desired ranges. Both slurry and slurry by-products become embedded within the pad and the pad material itself changes as a result of the mechanical and chemical influences. For certain fibrous pads, the napth of the pad may need refreshing. For other pads, the actual pad material may experience plastic deformation. In order to achieve a consistent surface quality, surface dressing is required. This is typically called ‘conditioning’ and is usually achieved by either raising the surface with an inert plastic brush or removing surface material via abrasive by a bound hard particle disk. In some cases, chemical treatment in the absence of mechanical dressing is used. The particular approach generally depends on the origin of the issue and the type of polishing pad.
FIG. 1 shows a cross sectional view of a dielectric layer undergoing a fabrication process that is common in constructing Damascene and dual-Damascene interconnects. The dielectric layer has a diffusion barrier/adhesion promotion layer (typically layers of TiN, Ta or TaN) deposited over the etch-patterned surface of the dielectric layer. Once the diffusion barrier layer has been deposited to the desired thickness, a copper layer is formed over the diffusion barrier layer in a way that fills the etched features in the dielectric layer. Some excessive diffusion barrier and metallization material is also inevitably deposited over the field areas. In order to remove these overburden materials and to define the desired interconnect metallization lines and associated vias (not shown), a chemical mechanical planarization (CMP) operation is performed.
As mentioned above, the metal CMP operation is designed to remove the top metallization material from over the dielectric layer. For instance, as shown in FIG. 2, the overburden portions of the copper layer and the diffusion barrier layer have been removed. As is common in CMP operations, the CMP operation must continue until all of the overburden metallization and diffusion barrier material is removed from over the dielectric layer. The greater the planarization efficiency of the metal CMP process, the less metal needs to be deposited, and the more efficiently a fully planar surface can be achieved. Depending on the slurry or slurries used, the copper and barrier film removal may occur using a single slurry with one or more process steps, or may be performed using slurries targeted for each layer in multiple steps. Due to the inherent across wafer and within die pattern dependent thickness variation associated with the deposition process, a given location on the wafer will clear first while another location will be the last to clear. As the goal is to remove all metal in the field region across the entire wafer (though minimize removal in the trench region), the portion of the wafer to first clear will receive relatively more overpolish than that portion of the wafer which is last to clear. This is termed overpolish. A more efficient planarization process can minimize the extent to which one portion of a die clears before another. Similarly, the greater the ability of the CMP process to minimize the induced topography during such overpolish, while maintaining suitable performance in the vast array of other metrics, the better the overall process performance.
Clearly, optimization of a CMP process is multifaceted. The process capability, stability and manufacturability all must be met. Within capability, both topography and defectivity are paramount. Having better pads increases the margin by improving the efficiency of planarizing and lowers the susceptibility to topography generation during overpolish. Stability with respect to pad amounts to pad life. Manufacturability with respect to the pad amounts to the cost of the pad as a consumable, which is related to pad stability via pad life, as well as process capability in terms of process line yield limitations due to associated pad contribution to process capability.
As indicated above, polishing pads for CMP applications can be classified into a number of categories. Fibrous pads have found commercial application in semiconductor applications. Such pads may consist of woven or nonwoven fibers. Additionally, the fibers may have applied coatings. An example of an impregnated fiber pad is the Rodel Politex, Rodel Suba series and the Thomas West pad (Kanebo). Fiber pads may even consist of voided regions as the coating on the fibers impinges on one another. An example of a pad using a hard coating is the recent offerings by PsiloQuest Inc, which uses a hard ceramic coating on an otherwise softer cellular substrate. An example of a filled pad is the 3M fixed-abrasive pad which has embedded ceramic particles. Such a pad is typically used with an abrasive-free reactive liquid, where the pad itself contributed the abrasive particle to the system, in contrast to slurry-based approaches in which the abrasive is suspended in the liquid chemistry. The pad is also made in a roll format. The prior art also discusses fully solid polymeric pad in Rodel U.S. Pat. No. 5,489,233 by Cook et al. In this single pad manufacturing approach, there is no internal void space and no intrinsic ability to either absorb or transport slurry.
There are a number of patents which address pads that do not constitute completely solid materials. These cellular pads include both open and closed cell forms. Cell size is an important pad characteristic in that it acts as a vehicle to absorb and transport slurry. While such pores are susceptible to plastic deformation and glazing as mentioned above, efficient replenishment of the surface can be achieved through abrasion of the surface material to yield new cells. With smaller cells, this replenishment can occur more frequently and efficiently, thus providing for a more stable CMP process and longer pad life.
A dominant issue within such art is the ability to control cell size. An example of the open cell structure is presented in Rodel U.S. Pat. No. 6,325,703 by Cook et al. The interconnectivity of an open cell structure allows for significant slurry and slurry by-product absorption, but generally requires an underlying layer to act as a moisture barrier and maintain adhesion integrity to the polishing platform. There are a number of closed cell formation approaches available in the prior art. Rodel U.S. Pat. Nos. 6,434,989, 5,900,164 and 5,578,362 by Reinhardt et al present closed cell materials through the use of incorporated polymeric microelements. In this cured and sliced cake approach, the incorporated microelements provide an exposed open void during use upon either direct mechanical shearing through the element during conditioning or through dissolution of the element shell during contact with the aqueous polishing liquid. The size of the microelements and the degree to which they coalesce directly translates to the effective closed cell size of the pad material. Other incomplete solid materials have been proposed as well. Polyurethane processed through water blown formation techniques are some of the earliest pad materials used for CMP processing. In this case, urethane reaction by-products include urea and CO2 gas, which becomes trapped within the viscous and curing urethane material. The drawback of such processing stems from the violent exothermic reaction and the limited control over this process. The Universal Photonics approach is some of the earliest work in which the uncontrolled reaction leads to a wide cell size distribution. Average cell sizes are typically 50 um diameter and above and include a significant tail in the distribution to very large cell size, in some cases leading to macroscopic holes in the pad. The Toray process cures the polyurethane under pressure at room temperature in an attempt to control cell size, but the resulting cell size is still large. The Poval process uses a very large closed container system, which is again cured from the inside out, with large cell size and wide size distribution resulting from the poorly controlled reaction release of CO2. In contrast to the water blown systems, JSR has incorporated water soluble fillers which, upon dissolution, give rise to a voided cellular structure.
Cell size formation can also be accomplished through direct gas injection into the polymeric materials. U.S. Pat. No. 3,491,032 (Skochdopole et al.; Jan. 20, 1970) describes a process for making cellular polymer materials. In a process of Skochdopole, finely divided solid materials such as calcium silicate, zinc stearate, magnesium stearate and the like can advantageously be incorporated with the polymer or gel prior to expanding the same. Such finely divided materials aid in controlling the size of the cells, and are employed in amounts of from about 0.01% to about 2.0% by weight of the polymer.
U.S. Pat. No. 5,116,881, issued to Park et al. on May 26, 1992, describes polypropylene foam sheets and a process for their manufacture. In a process of Park, a nucleating agent, is used to create sites for bubble initiation. It is preferred that the nucleating agent have a particle size in the range of 0.3 to 5.0 microns and that its concentration be less than one part per hundred parts polymer by weight. Concentrations of nucleating agents greater than five parts per hundred parts polymer by weight leads to agglomeration, or insufficient dispersion of nucleating substance so that the diameter of the cell size becomes greater.
Rogers Inouac and others mention dissolving gas into the liquid polymers under high pressure. This gas is susceptible to coming out of solution as a result of a pressure drop. The gas takes the form of bubbles.
In addition to cell size formation and control, a wide range of pad design attributes have also been addressed in the prior art including composition, pad flatness, texture, grooving, layer stacking, and porosity (distinguished from microcells). A number of issues feed into pad stability and include the chemical integrity of the material as a result of its direct exposure to the polishing liquid (moisture absorption, chemical reactions), pad material curing at room temperature, mechanical property issues such as elasticity, plasticity, anelasticity and associated glazing response.
The prior art teaches a wide range of material compositions for polishing pads which include at least one moiety from the following: a urethane; a carbonate; an amide; an ester; an ether; an acrylate; a methacrylate; an acrylic acid; a methacrylic acid; a sulphone; an acrylamide; a halide; an imide; a carboxyl; a carbonyl; an amino; an aldehydric and a hydroxyl.
U.S. Pat. No. 6,641,471 owned by Rodel states that slurry distribution is related to, and is conceivably required or assisted by, friction between the semiconductor and the wafer. It specifically states “Slurry is most usually introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.” However, the importance of controlling friction and surface friction to maintain integrity of the target films, particularly those comprising low k dielectrics which may be porous and extremely fragile, is not addressed. Research investigating the roll of the coefficient of friction (COF) has been performed and published by others, but they do not specifically target low COF processing, nor do they explicitly link such work to targeting low COF pad materials surfaces nor do they propose either micropores or the combination of low COF with micropores to achieve low friction during processing to avoid interfilm and intrafilm failure issues which occur during the polishing process, particularly in the regime of low k dielectrics. Tightly packed micropores act to minimize surface contact area. They also immediately provide local slurry supply and thus limit frictional effects and ameliorate excursive contact effects which can result in intrafilm and interfilm failures. By lowering the friction effects, a lower surface temperature can be achieved and the isotropic chemical etching component can be minimized, thereby increasing planarization efficiency.
The prior art also teaches many embodiments of surface texture. In most cases, the surface texture of the polish pad stems from an intrinsic microtexture as a result of its method of manufacture. The surface microtexture is derived from bulk non-uniformities which are deliberately introduced during manufacture of the pad. When cross-sectioned, abraded, or otherwise exposed, the bulk texture becomes a surface microtexture. This microtexture, which is present prior to use, permits the absorption and transport of slurry particles, and gives rise to polishing activity without further addition of micro- or macrotexture to the pad. Examples of such an approach include urethane impregnated polyester felts (examples of which are described in U.S. Pat. No. 4,927,432) in which the microtexture is derived from the ends of projecting fibers within the bulk composite, together with associated voids. Microporous urethane pads of the type sold as Politex by Rodel, Inc. of Newark, Del. have a surface texture derived from the ends of columnar void structures within the bulk of a urethane film which is grown on a urethane felt base. Filled and/or blown composite urethanes such as IC-series, MH-series and LP-series polishing pads manufactured by Rodel, Inc. of Newark, Del. have a surface structure made up of semicircular depressions derived from the cross-section of exposed hollow spherical elements or incorporated gas bubbles. Abrasive-filled polymeric pads such as those of U.S. Pat. No. 5,209,760 possess a characteristic surface texture consisting of projections and recesses where filler grains are present or absent. In yet another case, the surface microtexture is not introduced as a method of manufacture and is a result of in-situ generation during the CMP process. An example of such an approach is the OXP pad by Rodel. “Macrotextures”, or larger size textured artifacts including grooves, may be imposed on the work surface of a pad by embossing, skiving, perforating and/or machining, for example. In conventional polishing pads, the spacing and/or size of individual macrotexture artifacts or features is generally greater than 5 mm. The spacing and size of these artifacts are typically very regular and repetitive but more complex fractal patterns have been proposed.
The prior art reveals that pore size control is an important characteristics for most, if not all, pads. For those pads which do not have an intrinsic surface texture as a result of pad microstructure, nor induced macrostructure or grooves as a result of the pad manufacturing process, a microstructure is induced as part of the polishing system. While microcell creation has been achieved through multiple methods, the absolute average cell size has been generally limited to well above 10 microns, and the size distribution has been quite broad. A technique which allows for smaller cell size and tighter size control is warranted. The incorporation of microelements has allowed for reasonable cell size distribution, but at a larger average cell size than desirable. Additionally, cell size creation though microelements or microspheres which dissolves in aqueous environments, leaves a polish pad and process susceptible to pad by-product induced defectivity. A pad material, and process for manufacturing such, which provides for more uniform cell size and smaller cell size, which does not utilize the incorporation of a foreign material is desirable.
The present invention is directed to a polishing pad, and method of make the same, which provides for the incorporation of small uniform gas bubbles of controlled size and shape through a novel gas injection technique leveraged to an actuated reaction injection molding process. The improved process includes producing a molded microcellular elastomer with a reduced number of voids from at least two liquid reactants and a gas. The process includes introducing into at least one reactant a gas to form an admixture; passing the admixture through a static mixer at superatmospheric pressure; then immediately mixing the admixture with the other reactant at superatmospheric pressure to form a reaction mixture; introducing the reaction mixture into a mold in which the pressure is substantially below the superatmospheric pressures used above; and curing the reaction mixture in a mold to produce a molded microcellular elastomer with a reduced number of large voids.